In a typical cellular network, also referred to as a wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro NodeB, home NodeB or pico base station, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation, 3G, mobile communication system, which evolved from the second generation, 2G, mobile communication system GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Cellular networks evolve towards higher data rates, together with improved capacity and coverage. In 3GPP, standardization body technologies like GSM, High Speed Packet Access (HSPA) and LTE have been and are currently developed.
In the context of this disclosure, a base station as described above will be referred to as a base station or a RBS. A user equipment as described above, will in this disclosure be referred to as a user equipment or a UE. A radio network controller as described above will be referred to as a radio network controller or a RNC.
The expression DownLink (DL) will be used for the transmission path from the base station to the user equipment. The expression UpLink (UL) will be used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
To support mobility of user equipments between cells in a cellular network, so called handovers are be performed. A handover may be a change of serving cell, so that a user equipment being served by one cell, becomes served by another, so called candidate cell, instead. A handover may also mean addition of a radio link to a candidate cell without removing established radio links to other cells; or replacement of one of the existing radio links by a radio link to a candidate cell. Communication that involves a multitude of cells may be referred to as macro diversity, soft handover, softer handover or coordinated multipoint, etc. The set of cells with which the user equipment have radio links is referred to as the active set. Hence, a handover includes the setup of a radio link connection between the user equipment and the candidate cell, so that the candidate cell may become the serving cell for the user equipment.
Some cellular networks feature soft handover where a user equipment may be allocated a multitude of radio links. These radio links may be with the same base station or different base stations. Furthermore, the base stations may be controlled by different nodes, such as for example different RNCs.
Neighbor cell relations may be used to facilitate handover of the user equipment between base stations. In UTRAN, the user equipment is provided with a so called neighbor cell list comprising cell identifiers, or indexes associated with cell identifiers, of plausible candidate cells to search for and perform measurements on. The user equipment is provided with the neighbor cell list by its so called serving RNC, which is also the RNC to which it reports when a candidate cell is detected.
Each cell in the cellular network broadcasts a well-defined signature sequence. In UTRAN, this sequence is a scrambling code. Sequences are enumerated and associated with the cell identifier for the cell. The scrambling codes are reused in the cellular network, and are therefore not globally unique. They may however be perceived as locally unique by careful selection, and thereby they may be used by the serving RNC to unambiguously identify candidate cells for handover when the user equipment reports them. The user equipment regularly perform measurements to detect scrambling codes which are broadcast in its vicinity.
In addition to the reused scrambling codes, the base stations broadcasts globally unique cell identifiers, so called Cell Global Identifiers (CGI)s. However, in order to keep the candidate cell measurement procedures simple, these may not be decoded and reported to the serving RNC for candidate cell discovery purposes.
Because of scrambling codes being reused they may however in some situations not be locally unique, due to for example a large amount of cells being present in an area.
This is a problem since the serving RNC cannot then unambiguously determine which cell is actually reported by the user equipment when such a locally reused scrambling code is reported.